The present invention relates to a fluid pressure sensor, e.g. usable for a vehicle wheel in order to detect the inflation pressure of a pneumatic tire on the wheel.
In applications that require sensors of small size, it is known to have recourse to sensors made by etching plates of crystal material. One such pressure sensor, said to be of the resonant type and recognized as having performance that is better than ordinary micro-machined sensors comprising a diaphragm with strain gauges, comprises a box defining a housing, and at least one resonator received in the housing and suspended by flexible beams from at least one elastically deformable diaphragm closing the housing. The housing also contains resonator excitation means for setting the resonator into vibration and detector means for detecting a vibration frequency of the resonator. The deformable diaphragm thus interacts with the resonator in such a manner that deformation of the diaphragm under pressure from the surrounding fluid leads to a corresponding variation in the frequency of the resonator. The excitation means are arranged to set the resonator into vibration at its resonant frequency and the detector means are constituted for example by capacitive elements or by strain gauges that provide a control unit with a signal representative of the vibration frequency of the resonator. The control unit is arranged to deduce the pressure acting on the diaphragm from the frequency it detects.
It is found that many of those sensors present low sensitivity.
Capacitive detection is complex to develop and is thus less effective than strain gauge detection, at least in theory. In practice, strain gauge detection is found to be of relatively low performance.
In order to improve the sensitivity of sensors, the circuit of the control unit has been made more complex in order to process the signals delivered by the strain gauges. That has contributed to increasing the cost and the general size of the sensor but without that leading to a sufficient increase in sensitivity.